This application relates to and incorporates herein by reference Japanese Patent Application No. 9-328996 filed on Nov. 28, 1997.
Field of the Invention
The present invention relates to a position detecting device for detecting position of a moving body by the use of a magnetoresistive element (MRE), and particularly to a rotation detecting device used for engine control or braking control in a vehicle.
The ignition timing of an engine is determined on the basis of crankshaft angle position and camshaft angle position. For example in a four-stroke engine, because the camshaft attains one rotation for every two rotations of the crankshaft, cylinder identifying information is provided within one rotation of the camshaft and ignition timing information is provided in one rotation of the crankshaft.
Conventional rotation detecting devices use, as shown in FIGS. 9A and 9B, a bias magnet 102 to project a bias magnetic field toward a camshaft gear 101 to detect the rotation position of the camshaft 101 by detecting with an MRE 103 the direction of the field as it displaces on the basis of the ridges (gear tooth convexities) 101a and valleys (gear tooth concavities) 101b of the camshaft gear. 101 changing from ridge to valley and from valley to ridge.
In FIGS. 9A and 9B, it is shown respectively that the gear 101 is in a valley position and in a ridge position at the detection position in a static state immediately before the gear 101 starts to rotate, that is, on engine startup. In the figures the magnetic field projected toward the gear 101 by the bias magnet 102 is shown by arrows.
The orientation of the lines of magnetic force (direction of the magnetic field) passing through the MRE 103 is the same between cases when the gear 101 is in the ridge position and in the valley position. The output waveforms of the MRE 103 in these respective positions are shown in FIG. 10. During the period of from when the power supply is switched on until the first point of change (point of change from ridge to valley or a point of change from valley to ridge of the gear 101 (period T), the output waveforms of the MRE 103 are the same for both positions.
Consequently, in this period T it cannot detect whether the gear 101 is in a ridge position or in a valley position, and therefore the correct position of the gear 101, cannot be detected. Thus, it is impossible for the ignition timing, that is, which cylinder should be ignited, to be determined from the position of the gear 101, and consequently the;, first one of the cylinders to be ignited upon engine starting cannot be identified and is not ignited the first time.
It is therefore an object of the present invention to provide a position detecting device which can detect the position of a moving body from a static state immediately before the moving body starts to move.
According to a first aspect of the present invention, a resistive element group including at least one magnetoresistive element is disposed in a position offset in a moving direction of a gear of a moving body from the magnetic center of a bias magnetic field projected by a bias magnet. Thus, the direction of the bias magnetic field is made different when the gear position changes to a valley position from a ridge position. This change in the bias magnetic field is used to obtain an output of the magnetoresistive element unit. It is possible to obtain an output value that is different when the gear is in the valley position from when it is in the ridge position. It is possible to distinguish between the ridge position and the valley position of the gear from the static state immediately after power-on, i.e., before the moving body starts to move.
Preferably, there is also provided another magnetoresistive element unit. The first magneto resistive unit is made up of first and second magnetoresistive elements, and the second magnetoresistive element unit is made up of third and fourth magnetoresistive elements. The first magnetoresistive unit is disposed in a position offset from the magnetic center of a bias magnetic field in a moving, direction of a gear of a moving body. The second magnetoresistive element unit is disposed in a position offset from the magnetic center of the bias magnetic field in the opposite direction to the first including at least one magnetoresistive element. The outputs of the third and fourth magnetoresistive elements in addition to the first and second magnetoresistive elements are used to detect movement of the moving body.